feat: Updating integrations section

[valekjo]

This PR updates the integrations section.

• The homepage contains some general info and splits integrations into 4 categories.
• API section contains links to api reference and both clients and a section on tokens
• Services have their own pages, mostly "empty" or with embedded youtube videos
• Make and Slack pages were left as they were.
• Webhooks section was left as it was.

Also I've added .nvmrc, felt like it was missing.

Resolves #514

@mtrunkat Could you please check it content wise (if something important is not missing and the structure is ok)? I'll pass it for edits to content team later :)

[github-actions]

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[mtrunkat]

I love this :)

[mtrunkat]

I will do a full review on Monday, but dropping a few notes:

• Some migrations from https://apify.com/integrations are missing - for example, Hevo or Transposit ETLs
• At integrations home, we could display some cards as, for example we use here - feat: Redoing home and moving getting started to actors/running and actors/development sections #542 - they also support images so we can include logo

[valekjo]

I've incorporated the PR and cards and did following changes on top of it

• integrations we have described in help or elsewhere are linked directly from integrations home
• integrations we don't yet have materials for (but likely will have soon) are omitted

[valekjo]

@mtrunkat I think I can give this to marketing for edits now, right?

[mtrunkat]

I went thru and give it one more iteration, please check the PR for this PR out